Numerous apparatus for heat sealing plastics resin films are known. Some are intermittent sealers, providing spaced seals across a film web as the film web passes between sealing surfaces. Others are fin sealers, providing a continuous seal along the edges of superimposed film layers, employing either a pair of heated bands or a pair of heated rollers to provide heat to opposing sides of the layered structure. Still others are lap sealers, producing a tube from a single layer of plastics resin film by sealing the overlapped edges of the film to one another. It is to such lap sealers that the present invention relates.
One such lap sealer is described in U.S. Pat. No. 3,729,359. In this patent, a plastics resin film is drawn over a forming tube, with a product passing through the forming tube to be sealed within the tube of plastics resin film to be formed and a free-wheeling heated band travelling in contact with the overlapped edges of the plastics resin film. This band is heated along a portion of its length in contact with the plastics resin film, providing the lapped seal to the film, and is cooled along a subsequent portion of its length in contact with the plastics resin film to avoid sticking of the film to the band after sealing. There are notable shortcomings in this sealer.
First, since the sealing band is driven by the plastics resin film passing between it and the forming tube, it is impossible to vary the speed of the band, thus limiting use of this equipment to those films in which band speed control is not critical. Secondly, due to the tendency of sticking of the film, again, due to the lack of speed control for the band, a cooling element is needed adjacent the heater. Additionally, employing the forming collar as the back-up surface for sealing requires a relatively large forming collar and requires that whenever the size or shape of the tube to be formed changes this large and relatively expensive collar must be replaced in its entirety.
It is thus a primary objective of the present invention to provide a lap sealer for plastics resin film tubes which does not rely upon the film speed for its speed of operation. It is also a primary objective of the present invention to provide a band sealer for lap sealing plastics resin film tubes which eliminates the necessity for a cooling means for the band, thereby reducing energy usage both in the cooling operation and in the need to reheat substantially the cooled band. It is also a primary objective of the present invention to provide a lap sealer for plastics resin film tubes which does not employ the forming collar as a portion of the sealing mechanism, thus allowing the use of readily available commercial forming collars in conjunction with the lap sealing apparatus.